thunderbird: Update version from 52.5.0 to 52.6.0

The release notes are available [online][1].

[Several security issues were fixed.][2]

[1]: https://www.mozilla.org/en-US/thunderbird/52.6.0/releasenotes/
[2]: https://www.mozilla.org/en-US/security/advisories/mfsa2018-04/

thunderbird.be0

@@ -2,7 +2,7 @@
 
 #export BEE_TMP_TMPDIR=/dev/shm BEE_TMP_BUILDROOT=/dev/shm/bee-root BEE_MAKEFLAGS='-j80'
 
-# BEE_VERSION thunderbird-52.5.0-0
+# BEE_VERSION thunderbird-52.6.0-0
 
 SRCURL[0]="https://ftp.mozilla.org/pub/thunderbird/releases/${PKGVERSION}/source/thunderbird-${PKGVERSION}.source.tar.xz"